1. Field of the Invention
The present invention relates to a color CRT display apparatus having a compensating coil for generating a magnetic field to cancel an external magnetic field which influences on the color CRT in the tube axis direction.
2. Description of the Related Art
A cathode ray tube (hereinafter referred to as a CRT) for displaying a color image is called as a Braun tube, which is used in a television set and so forth. The CRT has an electron gun from which electron beams are emitted. The electron beams are deflected by a magnetic field formed by a deflection yoke. The deflected beams are made to scan a display panel so as to light phosphor coated portions on the display panel, resulting in images on the display panel. Because of this construction, the images are easily influenced by an external magnetic field. If an external magnetic field such as the earth magnetism is given to the CRT, the position at which the electron beam collides with the display panel is changed so that a problem arises in that a correct image can not be obtained.
To prevent the influence by the external field, in a conventional color CRT display apparatus, the CRT is magnetically shielded by covering the CRT with a magnetic tube. It is, however, impossible to provide the magnetic shield on the portion of the display panel of the CRT because the display panel is to be seen from the external. Therefore, it is necessary to prevent the magnetic influence of the external magnetic field in the tube direction connecting the display panel of the CRT and the electron gun.
FIG. 1A is a cross-sectional view of a conventional color CRT display apparatus provided with a compensating coil around a panel disclosed in, for example, the Japanese Utility Model Publication (kokai) No. 61-33574). In the illustrated conventional example, the magnetic field for cancelling the external magnetic field in the tube direction of the CRT is automatically generated by using the compensating coil. In the figure, reference numeral 1 is the CRT including an electron gun 1a for emitting electron beams, a panel 1b adapted to be irradiated by the electron beams to display images, and a funnel part 1c provided between the electron gun 1a and the panel 1b.
Reference numeral 2 is a magnet detection clement for detecting the polarity or the strength of the external magnetic field in the tube direction connecting between the electron gun 1a and the panel 1b to convert it into an electric signal. Reference numeral 3 is a current supplying circuit for receiving the electric signal from the magnet detection element 2 to supply a current proportional to the detected strength of the external magnetic field and having a direction corresponding to the detected polarity. Reference symbol 4a is a compensating coil for generating a compensating magnetic field for cancelling the external magnetic field in the tube direction of the CRT 1 when the current is supplied from the current supplying circuit 3. In the example shown in FIG. 1A, the compensating coil 4b is wound around the panel 1b of the CRT 1. Reference numeral 5 is a deflection yoke for deflecting the electron beams emitted from the electron gun 1a. The deflection yoke 5 is wound around the separator flange 6.
FIG. 1B is a cross-sectional view showing a color CRT display apparatus in which a compensating coil 4b is wound around the deflect ion yoke 5.
Next, the operation of the color CRT display apparatus shown in FIG. 1A or 1B will be described. The magnet detection element 2 detects the polarity or the strength of the external magnetic field, and converts the detected polarity or strength into an electric signal which is then outputted to the current supplying circuit 3. In response to the electric signal, the current supplying circuit 3 supplies a current proportional to the strength of the external magnetic field and having a direction corresponding to the polarity to the compensating coil 4a. Thereby, the compensating coil 4a always and automatically generates a compensating magnetic field having the same strength as and opposite polarity to the external magnetic field in the tube axis direction so that the external magnetic field in the tube axis direction can be cancelled. Accordingly, even when the placing position of the CRT 1 is changed so that the polarity or the strength of the external magnetic field such as the earth magnetism is changed, the compensating magnetic field generated by the above-mentioned compensating coil 4a is automatically changed according to the change of the external magnetic field, so that the external magnetic field in the proximity of the CRT 1 along the tube axis direction can be cancelled.
The influence of the generation of the external magnetic field in the tube direction of the CRT 1 on the image display is that the electron beams are rotationally moved in the periphery of the image display of the CRT 1. The rotational movement is analyzed to be divided into a rotation of the raster as a whole and a rotation of purity. FIG. 2A and FIG. 2B are diagrams showing the influence on the image display when an external magnetic field is generated in the tube axis direction. In the figures, states of image displays are shown in which an external magnetic field is applied to the CRT 1 in the tube axis direction directing from the side of the panel 1b to the electron gun 1a.
FIG. 2A is a diagram showing a raster rotation. The raster rotation is a rotation of a raster as a whole which is caused by largely deflecting the electron beams as a whole in the rotating direction by the external magnetic field. FIG. 2B is a diagram showing a purity rotation. The purity rotation is, as illustrated in the figure, the rotational movement (here, it is a rotation in the clockwise direction) of the electron beams 8 which have passed through a not-shown shadow mask with respect to fluorescent parts 7 by which the electron beams are erroneously landed on portions illustrated by dash curves in the figure. By this purity rotation, the electron beams 8 collide with only a part illustrated by slash lines of the fluorescent parts 7.
The functions of the compensating magnetic field generated by the compensating coil 4a or 4b shown in FIG. 1A or 1B on the image display differ depending on the wound position of the compensating coil 4a or 4b. When the compensating coil 4a is wound in the proximity around the panel 1b as shown in FIG. 1A, for example, the functions of both the raster rotation shown in FIG. 2A and the purity rotation shown in FIG. 2B are generated, but the function of the purity rotation is generated notably in comparison with the function of the raster rotation. By contrast, when the compensating coil 4b is wound in the proximity around the deflection yoke 5, the function of the raster rotation shown in FIG. 2A is generated, but the function of the purity rotation shown in FIG. 2B is scarcely generated. The above-mentioned difference of the functions has been acknowledged by experiments.
Since the conventional color CRT display apparatus is constructed as described above, as shown in FIG. 1A, only the single compensating coil 4a wound in the proximity around the panel 1b of the CRT 1 is used to compensate the external magnetic field in the tube direction of the CRT 1. The compensating coil 4a wound at this position generates, as mentioned above, the magnetic field for generating the function of the purity rotation notably rather than the function of the raster rotation. Therefore, only one can do is to obtain the best adjustment by selecting either the raster rotation or the purity rotation to be a target, or to obtain a compromise compensation of both. According to this conventional compensation, there is a problem in that a high accuracy compensation corresponding to a high resolution of recent CRTs can not be achieved.